PHOENIX, Arizona (AFP) - The death of a prisoner held in an open-air jail in ferocious temperatures has triggered a debate about Arizona's prisons, with activists decrying conditions as "medieval and barbaric."

Marcia Powell, a 48 year-old prostitute serving a 27-month sentence, died after being held in a cage without shade in temperatures of 108 degrees Fahrenheit (42 degrees Celsius).

Powell was held in the cell for just under four hours as she was being transferred to a mental health facility on May 19. She collapsed under the scorching desert sun and died in hospital the following day.

The state prison department has launched a criminal investigation.

"We're hoping (for results) at the end of the month," said Arizona Department of Corrections spokesman Barrett Marson. "She was only supposed to be out there for two hours."

Since Powell's death, all 233 similar cells in Arizona's 10 state prisons have been shut down. They've been in operation since the 1960s, according to Marson, and are used as temporary facilities for prisoners being transferred.

Arizona Department of Corrections director Charles Ryan said the prison system had failed Powell.

"It's our responsibility to ensure the care and custody of the inmate population," Ryan said in a statement.

"The death of Marcia Powell is a tragedy and a failure. The purpose of the investigation is to determine whether there was negligence and to remedy our failures."

"It's barbaric in the state of Arizona to put a dog outside in the heat without protection or water for any length of time.

"For the Department of Corrections to do that routinely to prisoners is an indication of how far we have to go in this country to learn about human rights."

Summer temperatures in Arizona can swing from the high 90s to more than 120 degrees (36 to 49 Celsius). A healthy adult male will die in five hours without water in 100-plus temperatures, according to desert survival experts.

Hamm has contacted the Department of Justice and the FBI to conduct an independent investigation into the circumstances of Powell's death, saying she doubts Arizona authorities can be trusted to hold an impartial inquiry.

"The big question for us is that the (department) is conducting its own investigation," Hamm said.

"We have so many problems with that in so many directions we can hardly articulate it ... There is some criminal liability here. They should not be investigating themselves ... It's unconscionable.

"There's no excuse for the governor not to step in and say this is unacceptable."

In December 2007 Hamm wrote to the director of the department of corrections about prisoners being locked in the cages in December without jackets or blankets.

Prison guards contacted the organization after witnessing inmates being placed in the cells all day for 10 days.

"The department calls them outdoor enclosures," Hamm said. "They're cages, like dog runs with chain link fence on the sides and top. They're tall enough for a human being to stand in, with no seats or chairs.

"We are suspicious about whether Marcia Powell was provided with water when she was in the cage."